The present invention relates to optical LAN devices including a master node and a plurality of slave nodes that are interconnected by an optical fiber cable for configuring a network, and, more particularly, to techniques for suppressing a network crash due to failure caused in the slave nodes.
Typically, a ring type optical LAN device includes a master node and a plurality of slave nodes that are interconnected by an optical-fiber cable for configuring a network. In this device, an optical signal is sent from the master node to the network, circulated through the slave nodes, and returned to the master node. However, if any one of the slave nodes fails, the optical signal cannot be passed to the slave node subsequent to the failed slave node, leading to a network crash.
To solve this problem, Japanese Laid-Open Patent Publication No. 11-313098 describes a technique for preventing a network crash even if part of the slave nodes fails. More specifically, in accordance with the technique, each of the slave nodes is provided with an optical bypass transmission line. An optical branching device is connected to the upstream end of each of the optical bypass transmission lines. An optical coupling device is connected to the downstream end of each optical bypass transmission line. Each of the optical branching devices branches an optical signal transmitted through the network to an optical signal directed to the corresponding optical bypass transmission line and an optical signal directed to the corresponding slave node. Each of the optical coupling devices couples the optical signal from the corresponding optical bypass transmission line with the optical signal from the corresponding slave node, generating a single optical signal.
If any one of the slave nodes fails, the master node isolates the failed slave node from the network in terms of the protocol. Further, when the master node sends an optical signal to the network, the optical signal is transmitted to the slave node subsequent to the failed slave node through the optical bypass transmission line corresponding to the failed slave node. This prevents a network crash from being caused by the failed slave node.
Each of the slave nodes has a light sending portion. The light sending portions are controlled to flash for generating an optical signal transmitted to the network. In the aforementioned publication, if any one of the slave nodes fails and the corresponding light sending portion is maintained as turned off, the failed slave node maintains the optical signal in a turned off state. In other words, the failed slave node cannot transmit the optical signal to the network. In this case, when the corresponding optical coupling device receives a normal optical signal from the associated optical bypass transmission line, the optical coupling device simply sends the signal to the slave node subsequent to the failed slave node. This allows the slave nodes subsequent to the failed slave node and the master node to read required information from the optical signal, which is transmitted through the network, without being affected by the failed slave node.
In contrast, if any one of the slave nodes fails and the corresponding light sending portion is maintained as turned on, the failed slave node continuously transmits an optical signal maintained in a turned on state. In this case, the corresponding optical coupling device superimposes the optical signal from the failed slave node on a normal signal received from the associated optical bypass transmission line. That is, the optical coupling device continuously transmits the signal held in the turned on state to the slave node subsequent to the failed slave node. This prevents the slave nodes subsequent to the failed slave node and the master node from reading required information from the signal transmitted through the network, resulting in a network crash.